Killing your buddy is one thing. Killing your buddy in CounterStrike certainly is not that thing. Unless you secretly want to kill your buddy, you would if nobody caught you, and you’re using CounterStrike to kill him vicariously.
Adultery etc. is a different thing because while adultery in games is as pretended as the game is, actually consenting to sexual pleasure derived from what one’s doing in a computer game is basically porn and that’s sinful precisely because of deriving that sexual satisfaction from anything else than lawful marital intercourse that is also open to life. Don’t need a computer for that. Even mentally stimulating oneself sexually, intentionally done, can go up to mortal sin. (Any sexual pleasure obtained from unreal porn is still real sexual pleasure, which is wrong to seek other than from real intercourse with one’s lawful spouse.)
Also, one could violate the 9th commandment by coveting his buddy’s World-of-Warcraft sword as opposed to the sword that hangs on the wall of his dining room. That’d be because the coveting would be for real. At least if one allowed it to go to the same level as coveting a real thing. (Some people are capable of that.) Probably the 6th in case of stealing as well.
Real anger would be as possible to have over a computer game (not pretended anger of some RPG character but some real anger that had events from the game as its foundation) as it is in a sports game in real life.
But some things may pose a challenge to your faith, in which case you should avoid them (advice I got from a dutiful Franciscan priest). Also, if something is too much for your sensitivity, you should consider dropping it for the sake of your inner peace (I’ve done that, there are games I’ve started but not finished, or skipped entirely on account of a disturbing screenshot).
You can compare games to theatre. There are people who can pretend to be Greek gods or pagan priests or whatever (and somebody had to play Judas, too). But not everybody can do that without a risk to his faith or without mental anguish. And I suppose not everybody can safely enjoy games in which he’s doing lots of immoral stuff for fun. I’d be worried about a person who needed that form of enjoyment, if not from a moral, then at least from a psychological point of view.
Additionally, games can sometimes provide the impulse to consider a certain action from a moral point of view, e.g. a political decision that you wouldn’t be taking in real life because you’re not in a position to. That can perhaps be good. But you need to remember that games are not life and annexing a country in Civilization is not the same as waging war in real life.
